KYIV, Nov. 21 - Armed men in masks blocked off the center of Ukraine's rebel-controlled city of Luhansk on Tuesday in what the rebel leader said was a revolt by supporters of a sacked regional police chief, Reuters reported.

The Organization for Security and Co-operation in Europe, which monitors the implementation of a much-violated ceasefire agreement with Ukraine, said on Tuesday it had observed military-style vehicles and armed men in central Luhansk.

The tension in Luhansk did not appear to have any direct connection to the conflict between the rebels and Ukraine's government, but President Petro Poroshenko called a meeting of his security and defense council late on Tuesday.

"The Ukrainian armed forces are ready for all developments to ensure the safety of civilians," Poroshenko said in a statement.

Residents of Luhansk, capital of the self-proclaimed Luhansk People's Republic (LNR), saw unidentified armed men blocking access to downtown streets on Tuesday morning, according to local media and a resident of the city.

The OSCE's monitoring mission posted photos on Twitter that showed lines of heavy military trucks and armored personnel carriers in central Luhansk.

The LNR's rebel leader, Igor Plotnitsky, blamed disgruntled supporters of Igor Kornet, his chief of police whom he said he had fired on Monday.

"The attempts by certain elements in the interior ministry in this way to challenge the decision .... (to remove Kornet) have gone beyond the bounds of what is acceptable," Plotnitsky said in a statement on his administration's web site.

"I can say with confidence that the attempts by certain persons to stay in power by destabilizing the situation ... are futile, and in the very near future will be neutralized."

But Kornet said he was still in his job, and demanded that senior figures in the region's rebel leadership be prosecuted.

"I want to dispel rumors that I've been removed," he said in a video message, clad in field camouflage. "We have the situation completely under control."

He said that he had uncovered evidence that several senior LNR officials, acting in collusion with Kiev, were "involved in criminal activity to the detriment of the interests of the republic and the people of Luhansk.Ф

"Last night I provided all the documents available to the ...Igor Plotnitsky, who decided to launch criminal cases and arrest those involved," Kornet said.

In his statement the Luhansk rebel leader contradicted this, saying there were no grounds to arrest the officials identified by Kornet.

The conflict in the country's east between Ukrainian government forces and pro-Russian separatists has killed over 10,000 people. Russia denies accusations from Ukraine and NATO that it supports the rebels with troops and weapons. (rt/ez)